The aging of the brain is a cause of cognitive decline in the elderly and the major risk factor for neurodegenerative diseases. An exciting recent development is the elucidation of a pattern of DMA damage in the aging human brain that is associated with reduced expression of genes that mediate synaptic plasticity, vesicular transport and mitochondrial function. Our finding of a "genetic signature" of brain aging that can be explained, at least in part, by oxidative DNA damage to vulnerable gene promoters provides a novel conceptual framework for understanding how the brain ages. Furthermore, we have begun to define the mechanism by which damaged genes are silenced by obtaining evidence for the involvement of a nuclear protein complex that contains the longevity gene Sirtl, the transcriptional co-repressor NCOR1, and the DNA repair enzyme hOGG1. Our preliminary studies also raise the possibility that age-related .DNA damage and gene silencing may predispose to the pathology of Alzheimer's disease. This hypothesis is further supported by the development of the Ck-p25 transgenic mouse model of Cdk5 dysregulation that shows markedly increased DNA damage and features of the pathology of Alzheimer's disease. These findings provide the basis for our hypothesis that DNA damage contributes to reduced expression of important neuronal genes in the aging brain, and that this process may underlie cognitive decline and vulnerability to neurodegeneration. The studies in this proposal will establish a genome-wide database of gene expression and DNA damage in the normal aging human brain, and will investigate the role of a newly defined DNA damage silencing complex involving the longevity gene Sirt 1. Transgenic mice that overexpress DNA repair enzymes will be generated and mated with: APPsw and Ck-p25 transgenic mouse models to determine the role of age-related DNA damage in the cognitive decline of aging and the pathology of Alzheimer's disease. Moreover, the role of impaired autophagy as a mechanism of oxidative DNA damage and protein aggregation in the aging brain will be investigated. Finally, a novel set of potentially therapeutic Sirtl-activating compounds will be tested in normal aging mice and in mouse models of human neurodegenerative diseases. These studies may provide new insights into brain aging, with potentially significant therapeutic implications.